Completed Project LM2576 with bypass Transistor for Higher Current [SOLVED]

Thread Starter

anishkgt

Joined Mar 21, 2017
549
Hello All,
What would be best way to get more current ? with this circuit i can see some N-type bypass transistors but not sure how to go about it ? i planing to charge a 2p3s array of caps which would be 206F@8.1v at about 40A. So from to charge from 0 to 8.1v, the math would be 206*8.1/40 = 42 Seconds to charge completely. What other changes would be needed to charge at these rates.

LM317 Charger.jpg
 

DickCappels

Joined Aug 21, 2008
8,686
Since it is a buck converter you can simply add an NPN emitter follower or N-channel source follower between the chip's output (pin 2) and the point where that pin is now connected -to the junction of the free-wheeling diode and the inductor.

Below is the technique used on a different buck regulator since sometimes pictures help.

1588289094859.png
 

DickCappels

Joined Aug 21, 2008
8,686
LM317: No noise from switching, not RFI problems, but dissipates considerably more power than...
LM22576: Dissipates relatively little power but there is more ripple on the power supply than the LM317 (without additional filtering)
 

Thread Starter

anishkgt

Joined Mar 21, 2017
549
LM317: No noise from switching, not RFI problems, but dissipates considerably more power than...
LM22576: Dissipates relatively little power but there is more ripple on the power supply than the LM317 (without additional filtering)
But when using the bypass mosfet like the SQP50N06 wouldn't all the current going via the mosfet ? so would it fine to keep the LM2256 a SOT225 package and the mosfte a through hole TO220 ?
 

Thread Starter

anishkgt

Joined Mar 21, 2017
549
yes i notice that the Q1 source should be connected before the inductor at the OUT pin of U1 but wouldn't that limit the current above 1 Amp ?
 

ronsimpson

Joined Oct 7, 2019
2,005
Again what is the Vcc voltage? How much current from the Vcc supply? 40A? Is this supply current limited?
Is the "40A" chosen because that is the max current the capacitors can handle?
Is the "40A" chosen because that is the max current from the Vcc supply?
 

Thread Starter

anishkgt

Joined Mar 21, 2017
549
Again what is the Vcc voltage? How much current from the Vcc supply? 40A? Is this supply current limited?
Is the "40A" chosen because that is the max current the capacitors can handle?
Is the "40A" chosen because that is the max current from the Vcc supply?
The Vcc is supplied by a server power supply limited to 100A. Not calculated the max current rating of the CAP. I've only limited the voltage.
 

DickCappels

Joined Aug 21, 2008
8,686
yes i notice that the Q1 source should be connected before the inductor at the OUT pin of U1 but wouldn't that limit the current above 1 Amp ?
Well, it would limit the gate current or base current for the external pass device to 1 amp but for a MOSFET that is approximately 1 amp x infinity and for a transistor 1 amp x Beta.

Which brings up something I overlooked. In the original circuit the current is limited inside the chip and when using an external pass element to boost the output current capability it is a good idea to limit the current gain of that pass element so that the 1 amp x Beta does not exceed a safe value for the pass element. A base-emitter capcaitor would do the trick in an emitter follower. In the case of a source follower a gate-to-source resistor should do it. If using a MOSFET I would test different gate-source resistor values until I found the right one. In the base of a bipolar transistor I would just assume that the transistor turns on when the IR drop exceeds VBE.
 

Thread Starter

anishkgt

Joined Mar 21, 2017
549
Ok a bit confusing, based on #15 how does it maintain a 8.2v ? Shouldn't Pin 2 of 2576WT be connected to the inductor as well ?
 

RIKRIK

Joined Oct 11, 2019
146
Ok a bit confusing, based on #15 how does it maintain a 8.2v ? Shouldn't Pin 2 of 2576WT be connected to the inductor as well ?
Okay imagine you have a vertical water pipe with water comming into the bottom off the pipe. When the water comes into the pipe, you dont want it over flowing out the top. So you put a tap in at the bottom aswell.

Now say if you wanted to maintain the water level half way up the pipe. This is your voltage regulation. You would have to turn the tap on and off. That all the ic is doing, on and off from the feedback it gets from the output.
 

Thread Starter

anishkgt

Joined Mar 21, 2017
549
My concern was if the Vcc is between 12v, how would it change R1 and R2 to get 8.2v ? The current values of R1 and R2 were based on the default configuration in the datasheet. So adding the mosfet has pin 2 connected to gate and now the FB voltage would be depended on the Vcc when the MOSFET is on. Does the Vout formula change then ?
 

RIKRIK

Joined Oct 11, 2019
146
No im not a expert as never used that component but looking at Dicks schematic of the ic.

I believe the two resistors are working as a voltage divider. These resistors will tell the ic when the output is going too high, which then turns pin 2 or anything connected to it off, then on and so on regulating the voltage. Like turning the tap on and off.

The only thing the resistors are looking for is that specific voltage. So it wont matter if there is a change in the input. You will notice that the ic can have wide inputs as voltage ranges. So unless specified in the data sheet it will be fine.

So the two resistors are like a program. Its sets the output voltage. Also the resistors have the advantage of limiting the current to the fb pins on the ic.

Heres a simple example http://www.learningaboutelectronics.com/Articles/Voltage-sensor-circuit.php

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